In conventional scanning CRT systems, misregistration between various different sequential horizontal scan lines in a video picture frame may occur due to differences in the amount of average beam current (intensity) drawn by each of the different horizontal scan lines. Typically the amount of horizontal misregistration is relatively small and goes unnoticed unless graphics data having known vertical orientations are to be produced. In such cases, the horizontal misregistration may still not appear excessive unless even block interlace excitation of graphics block letters is to be provided wherein the effect is magnified, especially for conditions where reverse video (black letters on a white background) is produced.
The horizontal misalignment between different average current scan lines is typically caused by variations in the CRT high voltage signal which is produced. These variations exist because for different average beam currents for each horizontal line a variation in the high voltage of the CRT screen may be produced. In order to prevent this problem it may be possible to maintain the high voltage at a fixed level by utilizing an extremely large, expensive capacitor connected to the high voltage. However, this solution is not practical from a cost standpoint, and therefore minor misregistration of different intensity (different average current) horizontal scan lines has been tolerated, but generally interlace excitation, particularly for even block graphics, was avoided since this magnified the horizontal misregistration effect.
It is believed that a prior "Conrac" system did provide horizontal compensation of an entire video display in response to overall video brightness changes by use of a series resistor between the power supply terminal of video brightness circuits and the CRT high voltage transformer. However, this was done just to maintain the horizontal picture size and this just occurred in response to changes in the overall video picture brightness. This prior compensation did not provide any horizontal orientation compensation between different horizontal lines, in a single video display, having different average video intensities.